i used a single pole dual throw switch to divert power from the coil to the horn.
that way if someone tries to start the car without flipping the switch first they will sound the horn.
until the switch is turned so power goes back to the coil it will never fire up.
best bit about it is its quite possibly the cheapest option.

a good alarm sytem is the toad ai606 which works very well with central locking.

I normally just remove the rotor arm to stop chancers if I'm leaving the car for any length of time. 5 second job and fits on your pocket, car ain't going anywhere then unless towed and you can't stop that. Mind you might have a look at one of those trackers.

i used a single pole dual throw switch to divert power from the coil to the horn.
that way if someone tries to start the car without flipping the switch first they will sound the horn.
until the switch is turned so power goes back to the coil it will never fire up.
best bit about it is its quite possibly the cheapest option.